1. Field of the Invention
The present invention relates to a vertical incidence type vertical optical modulator grown on a substrate, device driving method and fabrication process thereof.
2. Description of the Related Art
The following is an example of prior art semiconductor laser structures (Kasahara et. al., Japanese Patent Kokai Publication No. 5-152674 (see FIG. 12)). In FIG. 12, reference numeral 101 designates a semi-insulative GaAs substrate; 102 designates an Si-doped mirror having alternating laminated layers of n-GaAs and n-AlAs; 103 designates an undoped Al0.25Ga0.75As layer; 104 designates an undoped InGaAs active layer; 105 designates an undoped Al0.25Ga0.75As layer; 106 designates a Be-doped mirror having laminated layers of p-GaAs and p-AlAs; 107 designates a p-GaAs (3 xcex thick) layer; 108 designates a optical absorption layer composed of three InGaAs strain quantum well layers and a GaAs barrier layer (46 nm thick); and 109 designates an undoped GaAs layer (xcex thick). Reference numeral 110 designates an Si-doped mirror in which n-GaAs layers (xcex/4 thick, n=2xc3x971018 cmxe2x88x923) and n-AlAs layers (xcex/4 thick, n=2xc3x971018 cmxe2x88x923) are alternately laminated in 24.5 cycles. Reference numeral 111 designates a SiN film, 114 and 112 designate an n-side electrode and 113 designates a p-side electrode.
The device is operated under the condition where the n-side electrode 114 is grounded and a positive voltage is applied to the p-side electrode 113 whereas a negative voltage being applied to the n-side electrode 112. Since an electric field is applied to the optical absorption layer 108 which is reversely biased, the band end is shifted to longer wavelengths owing to the Franz Keldysh effect and light from the InGaAs active layer 104 can be turned ON and OFF. By virtue of the arrangement in which the optical absorption layer 108 is disposed between the Be doped mirror 106 and the Si-doped mirror 110, nearly 100% absorption can be accomplished at the time of voltage application even though the actual film thickness of the optical absorption layer 108 is only 30 nm.
In the prior art, since the absorption coefficient of the optical absorption layer 108 is only 1,000 to 10,000 cmxe2x88x921, the extinction ratio is increased by letting light reflect and reciprocate between the Be-doped mirror 106 and the Si-doped mirror 110. This approach, however, has presented the problem that the intensity of the out-going light is significantly low because the light modulated by the light absorption layer 108 reflects at the Si-doped mirror 110.
Other known vertical optical modulators are disclosed in Japanese Patent Kokai Publication No. 7-307464 (particularly in the paragraph No. 0010) and Japanese Patent Application No. 2000-275692.
The present invention has been made with the object of overcoming the foregoing problems.
This object can be accomplished by a vertical optical modulator according to the invention comprising: a first reflective layer having a quantum well structure formed on a substrate; a refraction index adjusting layer having a quantum well structure formed on the first reflective layer; a second reflective layer having a quantum well structure formed on the refraction index adjusting layer; a first electrode electrically connected to the underside of the refraction index adjusting layer; and a second electrode electrically connected to the upper surface of the refraction index adjusting layer,
wherein a bandgap of the quantum well structure of the refraction index adjusting layer is larger than those of the quantum well structures of the first and second reflective layers.
The first and second reflective layers may have a quantum well structure wherein metal dots are regularly, three-dimensionally arranged.
The metal dots may be gold dots and the refraction index adjusting layer may have a quantum well structure including InP.
According to the invention, there is provided a vertical optical modulator comprising:
a first semiconductor layer of a first conductivity type;
a second semiconductor layer of a second conductivity type joined to the first semiconductor layer;
a third semiconductor layer of the second conductivity type;
a dielectric layer formed between the second semiconductor layer and the third semiconductor layer;
an antenna electrode having a plurality of conductive pieces which are formed within the dielectric layer so as to have a net-like shape as a whole, to be separated from one another at the intersections of the net-like shape, and to be in contact with both the second semiconductor layer and the third semiconductor layer;
a first electrode electrically connected to the first semiconductor layer; and
a second electrode electrically connected to the third semiconductor layer.
Preferably, the width of the conductive pieces is substantially one third the spacing between the conductive pieces in a width-wise direction thereof.
Preferably, where the wavelength of a signal light modulated by the vertical optical modulator is xcex, the width of the conductive pieces is substantially xcex/3 and the spacing between the conductive pieces in a width-wise direction thereof is substantially xcex.
The plurality of conductive pieces may have the form of a lattice net as a whole.
The dielectric layer may be an air layer.
Preferably, concentration of an impurity of the second conductivity type of the second and third semiconductor layers is lower than that of an impurity of the first conductivity type of the first semiconductor layer.
The first semiconductor layer may be formed on a semiconductor substrate.
The semiconductor substrate may be semi-insulative and the first electrode may be formed on the first semiconductor layer whereas the second electrode being formed on the third semiconductor layer.
The semiconductor substrate may be electrically conductive and the first electrode may be formed at the underside of the substrate whereas the second electrode being formed on the third semiconductor layer.
The semiconductor substrate may be of the first conductivity type.
In a plan view, the first semiconductor layer, the second semiconductor layer, the antenna electrode and the third semiconductor layer have the form of a circle substantially having a first diameter, and either one of the first and second electrodes has the form of a circular ring having an outer diameter substantially equal to the first diameter whereas the other one of the first and second electrodes has an opening having a diameter substantially larger than the first diameter, and the first semiconductor layer, the second semiconductor layer, the antenna electrode, the third semiconductor layer, the first electrode and the second electrode are coaxially disposed.
The first conductivity type may be the n-type, whereas the second conductivity type may be the p-type.
The antenna electrode may be made from a high melting point metal.
According to the invention, there is provided a process of fabricating a vertical optical modulator, the process comprising:
a first crystal growth step of epitaxially growing, on a first semiconductor substrate, a first semiconductor layer of a first conductivity type and a second semiconductor layer of a second conductivity type in this order;
a first electrode formation step of forming an antenna electrode on the second semiconductor layer by vapor depositing an electric conductor film on the second semiconductor layer and performing lift-off, the antenna electrode having a plurality of conductive pieces which have a net-like shape as a whole and are separated from one another at the intersections of the net-like shape;
a second crystal growth step of epitaxially growing a third semiconductor layer of a second conductivity type on a second semiconductor substrate;
a substrate fusion bonding step of fusion-bonding the first and second semiconductor substrates by bringing the surface of the first semiconductor substrate on which the first electrode formation step has been performed into close contact with the surface of the second semiconductor substrate on which the second crystal growth step has been performed and by maintaining the first and second semiconductor substrates at 400xc2x0 C. or more for one or more minutes;
an etching step of removing the second substrate from the product obtained by the fusion bonding; and
a second electrode formation step of forming, after the etching step, a first electrode electrically connected to the first semiconductor layer and a second electrode electrically connected to the third semiconductor layer.
According to the invention, there is provided a vertical optical modulator wherein the first semiconductor layers of the first conductivity type, MQW layers having a semi-insulative quantum well structure and the second semiconductor layers of the second conductivity type are repeatedly laminated in this order; a first connecting region of the first conductivity type is formed such that parts of the ends of the repeatedly laminated first semiconductor layers are connected to one another; a second connecting region of the second conductivity type is formed such that parts of the ends of the repeatedly laminated second semiconductor layers are connected to one another; a semi-insulative third connecting region is formed such that the ends of the repeatedly laminated MQW layers are connected to one another and the third connecting region is interposed between the remaining parts of the ends of the first semiconductor layers and the second connecting region and between the remaining parts of the ends of the second semiconductor layers and the first connecting region; a fist electrode is formed in contact with the first connecting region; and a second electrode is formed in contact with the second connecting region.
In a vertical optical modulator according to the invention, a lower contact layer, a cross-band absorption layer having a quantum well structure that includes a hexagonal semiconductor layer, and an upper contact layer are formed on a substrate in this order, and a first electrode and a second electrode are formed so as to be electrically connected to the lower contact layer and the upper contact layer, respectively.
In a vertical optical modulator according to the invention, a cross-band absorption layer having a quantum well structure that includes InP is disposed on a substrate, and a gold dot layer is formed such that gold dots are dispersed on the upper surface of the cross-band absorption layer, and a first electrode and a second electrode are formed so as to be electrically connected to the lower surface and upper surface of the cross-band absorption layer, respectively.
These objects as well as other objects, features and advantages of the invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.